2014

Nicola Cochrane

E3HNDELECT/F142A

11/11/2014

Switchgear and Protection of HV Systems

INTRODUCTION

Page 1

Introduction

Switchgear and protection methods play a vital role in every high voltage system.

Electrical switches, fuses and other devices aim to rid the system of faults in minimal

time, minimizing the damage caused to equipment and the cost of repairs.

Dependability and security of protection must be reliable in order to safeguard the

entire system, ensuring continuity and security of supply as well as liability and the

safety of personnel in electricity transmission and distribution systems.

This project will focus on four types of switchgear and protection, primarily the

operation, application and constructional features of a high voltage fuse and

subsequently, the same aspects of a high voltage switch.

It will then proceed to look at two circuit breakers and describe the operation of a

Distance Protection Scheme, giving an example of a possible application. To conclude,

the operational features and applications of various types of Transformer protection

including restricted earth fault, Buchholz and winding temperature will be detailed.

INTRODUCTION

Page 2

Table of Contents

Introduction ________________________________________________________________ 1

High Voltage Fuse ___________________________________________________________ 3

Applications ___________________________________________________________________________ 3

Constructional Features __________________________________________________________________ 3

Operation ____________________________________________________________________________ 4

High Voltage Switch__________________________________________________________ 6

Applications ___________________________________________________________________________ 6

Constructional Features __________________________________________________________________ 6

Operation ____________________________________________________________________________ 7

Sulphur Hexafluoride (SF6) Circuit Breaker _______________________________________ 8

Vacuum Circuit Breaker _______________________________________________________ 9

Distance Protection Schemes _________________________________________________ 10

Translay Feeder Protection System__________________________________________________________ 10

Transformer Protection ______________________________________________________ 11

Restricted Earth Fault ___________________________________________________________________ 11

Buchholz ____________________________________________________________________________ 12

Winding Temperature ___________________________________________________________________ 12

References________________________________________________________________ 13

HIGH VOLTAGE FUSE

Page 3

High Voltage Fuse

APPLICATIONS

A high voltage fuse is a protective device utilized in high voltage power systems from

1.5kV to 138 kV. Typical applications of a high voltage fuse are:

Protection of an instrument transformer used for metering electricity, or

Protection of a small power transformer where the expense of a circuit breaker

is unjustified.

This project will focus on the following type of high voltage fuse:

HRC (High Rupture Capacity)

CONSTRUCTIONAL FEATURES

As demonstrated in figure 1.0, a typical HV HRC fuse consists of a metallic strip or

wire fuse element with a series of constrictions and a small cross-sectional area

compared to that of the fuse. The element is encased in a non-combustible, insulating

porcelain tube and enclosed at each end by an electrical terminal cap.

The terminal caps are conductive and the fuse is arranged in series. The flow of

current causes heat to be produced from the element due its resistance. Heat in turn

creates increased resistance and vice versa.

The eutectic fuse element is designed to carry a specific load current without

corrosion. In the event of a fault current which exceeds the normal load current, the

fuse element will vaporize, creating an open circuit.

A paramount part of any HRC fuse is the high grade refined silica sand. As current

flows through the fuse element, the sand conducts the heat away from the fuse

element, towards the durable outer fuse body. Thus, preventing any unwanted thermal

damage to the element. In addition and most importantly of all, in the event of a fault

current, the sand acts as a current limiting, arc-quenching medium by extinguishing

the arc internally.

Another feature found in many high voltage current limiting fuses is the striker

mechanism. This device is centered in the fuse and attached to one terminal cap, with

the other end of the striker pin connected electrically to the opposite terminal end by

HIGH VOLTAGE FUSE

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means of a highly resistive wire. Under fault condition, the fuse elements will melt and

the current will then jolt into the resistive striker wire, forcibly releasing the striker pin

either by method of a robust spring or the ignition of a pyrotechnic charge inside the

striker. The striker pin pierces the terminal cap. This can be a visual indicator that the

fuse needs replaced but is also frequently used to activate a tripping mechanism which

de-energizes all three phases connected to a switch. This process lasts only 0.1

seconds.

Figure 1.0

1. Terminal cap 5. Coil resistance

2. Porcelain tube 6. Striker device

3. ARC-perished filler 7. Star-shaped framework

4. Melting body

OPERATION

Under normal operating conditions, a high voltage HRC fuse is designed to carry the

rated current of the fuse void of any deterioration, for a period of time equal to or

greater than the lifespan of the other equipment in the system.

In the event of a low level fault current, the process of temperature rise in the fuse

element is considerably slow. This prevents the alloy-applied element constrictions

from melting simultaneously as they should. Instead the temperature increases

disproportionately, causing only a few breaks in the element and failing to operate the

striker mechanism. These breakages result in a slightly increased value of impedance,

incapable of quenching an arc. To overcome this problem, a minimal value of fault

HIGH VOLTAGE FUSE

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current which can be safely disrupted by a fuse has to be pre-determined by each fuse

manufacturer.

Should a high level short circuit occur the fuse will trip in a specific time. This involves

both the formation of many small electric arcs at the constricted points of the fuse

element and the operation of the striker mechanism. The arcs are quenched rapidly

through absorption by the silica sand filler which is packed tightly into the fuse and

solidifies into a powerful, insulating amalgam of glass and partly fused sand, called

fulgurite. This causes a dramatic rise in circuit impedance, causing the current to drop

to zero and in turn, creates a voltage spike.

HIGH VOLTAGE SWITCH

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High Voltage Switch

Essentially mechanical devices that conduct electrical current and provide an open

point in a circuit for isolation, all high voltage switches are incapable of disrupting fault

currents and must coordinate with system requirements and BIL (Basic insulation

level).

APPLICATIONS

Two applications of such high voltage switches are:

Disconnect switch – a switch capable of disrupting overload currents, utilized in

substations either on a line or on both sides of equipment for complete isolation

purposes.

Air break switch – also utilized as isolators but only in external substations,

these switches are fitted with an arc-perishing device such as an arcing horn

which quenches arcs formed during current interruption by extending the arc

beyond a point at which it can sustain itself. This feature enables this type of

switch to be operated whilst in an energized state.

CONSTRUCTIONAL FEATURES

Disconnect switches such as the one shown in figure 2.0 are simple in construction.

They consist of one conducting blade which is hinged at one end and stationary at the

other. The stationary part of the conducting blade can sometimes be fitted with a

mechanical hoop and a lock applied when in the closed position to prevent accidental

opening by extreme fault currents or bad weather conditions. Each terminal is

mounted on an appropriate insulator which must be capable of enduring voltage

surges and have good compatibility with the other system components in order to

operate effectively and efficiently. These insulators are selected by reference to BIL.

HIGH VOLTAGE SWITCH

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Figure 2.0

OPERATION

Under normal operating conditions, the disconnect switch will remain in a closed

position, conducting electrical current continuously without overheating.

In order for maintenance and repairs to be completed, the circuit requiring service

must first be de-energized. It is only after this step that the switches can be operated

safely. The switch can be operated manually through use of an insulated ‘hot stick’

which can be used safely from a distance to disengage the conducting blade from the

terminal, ascertaining complete isolation of that particular part of the system.

SULPHUR HEXAFLUORIDE (SF6) CIRCUIT BREAKER

Page 8

Sulphur Hexafluoride (SF6) Circuit Breaker

These circuit breakers operate in SF6 gas and are widely utilized in external electricity

transmission and distribution substations today. The high insulating properties of the

gas and exceptional arc-quenching ability deems it an effective method of protection.

The circuit breaker consists of one large chamber that houses two fixed contacts

separated by a specific gap which acts as a vacuum. There are two separate

moveable cylinders which contain the SF6 gas, one containing a fixed piston which is

connected to other stationary points in the breaker. Due to the positioning and vertical

movement of the cylinders, the internal volume of them will vary according to the state

of the circuit breaker. As demonstrated by figure 3.0, the breaker opens by moving the

cylinders down towards the pistons; this reduces the internal volume of the cylinder

and compresses the gas in preparation to quench an arc. Each cylinder has vents,

which in a closed position would be blocked by the stationary contacts. As the

cylinders continue to move down, more vents become unblocked and SF6 is forcibly

released through the vents and into the vacuum of the circuit breaker where the arc is

extinguished. In a closed position, the cylinder moves upwards so the internal volume

increases which in turn reduces the pressure of the gas. A disadvantage of SF6 circuit

breakers is SF6 gas as it is a potent greenhouse gas that is 20,000 times more harmful

to the environment than C𝑂2.

Figure 3.0

VACUUM CIRCUIT BREAKER

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Vacuum Circuit Breaker

Mainly utilized in medium voltage applications, a vacuum circuit breaker consists of a

heavy glass or ceramic vacuum chamber which houses a pair of electrical contacts;

one stationary, one moveable. There is a metal vapour condensing shield mounted

inside the chamber, providing dielectric protection from metallic vapour to the

equipment. The moveable contact is connected to the main control device by flexible

metallic bellows. This feature provides a seal for the vacuum, eradicating the

possibility of a leak. The vacuum acts as the arc-quenching medium and is deemed

more efficient than any other form of interrupter due to its extremely high insulating

properties. When the contacts are open, the ionization of metal vapour on the contacts

cause an arc to form. The arc is extinguished promptly due to rapid condensation on

the surface of the contacts of the electrons, ions and metallic vapour emitted by the

arc. Advantages of vacuum breakers include the minimal amount of damage caused to

contacts during operation and a maintenance free lifespan for approximately three

decades. See figure 3.1 for construction.

Figure 3.1

DISTANCE PROTECTION SCHEMES

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Distance Protection Schemes

Distance protection schemes utilize impedance relays to detect voltage and current

and are most suitable for applications with high speed auto-reclosing for the protection

of vital transmission lines over a distance of approximately 10 miles. There are several

advantages of distance protection including; high economical and technical factors,

and provision of both primary and remote back-up functions in one scheme.

TRANSLAY FEEDER PROTECTION SYSTEM

Under normal operating conditions, there are two current transformers in this system,

one at each end of the feeder as demonstrated in figure 4.0 Both CT’s carry equal

current so have the same polarity and are connected by a closed loop through two

relays via a loop pilot wire. The current flows through the loop so the potential

difference will be equal to zero (C𝑇2- C𝑇1= 0).

In the event of a fault, the current measurements will differ causing a resultant to pass

through the relay coils and trip associate circuit breakers in order to isolate the fault

line.

Figure 4.0

TRANSFORMER PROTECTION

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Transformer Protection

RESTRICTED EARTH FAULT

Restricted earth fault relays only operate in the event of an internal fault at the

star/neutral point of the system.

As demonstrated in figure 4.1, there are four current transformers in this protection

scheme; one for each phase and the neutral line. The secondary CT’s of each phase

of a power transformer are connected together in series then connected to the

common terminals of a secondary neutral CT to form a relay. This arrangement causes

the unbalanced secondary current from each phase to oppose the unbalanced

secondary current of the neutral. If the potential difference between the two currents is

zero and there is no resultant, then the currents are of equal amplitudes. However, if

the two current values differ, the REF relay will be activated due to the flow of the

resultant current.

Figure 4.1

TRANSFORMER PROTECTION

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BUCHHOLZ

A Buchholz protection relay is gas-operated and located, encased and oil-filled on the

pipe connecting the oil-filled conservator to the main oil-filled transformer tank. In the

upper part of the Buchholz container there is a hinged float connected to a mercury

switch which changes position according to the level of oil. In the lower area there is a

hinged baffle plate connected to a mercury switch. In the event of an internal fault such

as deterioration of insulation between windings, the oil surrounding the transformer will

decompose into hydrocarbon gases. These gases will rise up to the top of the

Buchholz container gradually reducing the oil level. This reduction in oil level causes

the float to move, which changes the position of the mercury switch, closing its

contacts and triggering an alarm.

WINDING TEMPERATURE

The lifespan of transformer windings is completely dependent on winding temperature.

If excessive, heat causes rapid deterioration of winding insulation and eventually

windings, dramatically reducing their lifespan. In avoidance of this, the temperature of

the windings has to be monitored consistently. This is achieved by implementing

current transformers to take measurements of current in order to calculate the

temperature. Relays can be connected to probes and placed in oil tanks to monitor

temperature, activating an alarm if a temperature difference occurs.

REFERENCES

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References

Youtube.com (May 2013), Industrial Fuses, 2nd November 2014,

http://www.youtube.com/watch?v=sqcaRamjr4M

tucsontransformer.com (1995-2012), Southern States Hook Stick Disconnect Switch,

7th November 2014,

http://www.tucsontransformer.com/img/southern-states-hook-stick-disconnect-switch-

05-1002-1.htm

electrical4u.com (No date specified), S𝐹6 Circuit Breaker, 7th November 2014,

http://www.electrical4u.com/types-and-operation-of-sf6-circuit-breaker/

electroniclevel3.wikispaces.com (No date specified), HRC Fuses, 5th November 2014,

http://electroniclevel3.wikispaces.com/HRC+Fuses

openelectrical.org (No date specified), Circuit Breakers, 11th November 2014,

http://www.openelectrical.org/wiki/index.php?title=Circuit_breakers

electrical4u.com (No date specified), Protection of Lines or Feeder, 11th November

2014,

http://www.electrical4u.com/protection-of-lines-or-feeder/

electrical4u.com (No date specified), Restricted Earth Fault Protection of Transformer |

REF Protection, 11th November 2014,

http://www.electrical4u.com/restricted-earth-fault-protection-of-transformer-ref-

protection/

REFERENCES

Page 14

WEEDY, B.M, CORY, B.J, JENKINS, N, EKANAYAKE, J.B & STRBAC, G. (2012)

Electric Power Systems. John Wiley & Sons: The Atrium.